1. Field of the Invention
This disclosure pertains to assemblies of ion optics and more particularly to assemblies of ion optic elements on an optical bench for mass spectrometer systems. 2. Description of the Related Art
Mass spectrometry is widely used in many applications ranging from process monitoring to life sciences. Over the course of the last 60 years, a wide variety of instruments have been developed. The focus of new developments has been two fold: (1) a push for ever higher mass range with high mass resolution, and (2) on developing small, desktop mass spectrometry instruments.
Mass spectrometers are often coupled with gas chromatographs for analysis of complex mixtures. This is particularly useful for analysis of volatile organic compounds (VOCs) and semi-volatile organic compounds (semi-VOCs). A combined gas chromatograph and mass spectrometer or spectrograph (GC/MS) instrument typically includes a gas inlet system, which may include the gas chromatograph portion of the GC/MS instrument. The GC/MS instrument typically also includes an electron impact (EI) based ionizer with ion extractor, ion optic components to focus the ion beam, ion separation components, and ion detection components. Ionization can also be carried out via chemical ionization.
Ion separation can be performed in the time or spatial domain. An example for mass separation in the time domain is a time of flight mass spectrometer. Spatial separation is seen in commonly used quadrupole mass spectrometers. Here the “quadrupole filter” allows only one mass/charge ratio to be transmitted from the ionizer to the detector. A full mass spectrum is recorded by scanning the mass range through the “mass filter.” Other spatial separation is based on magnetic fields where either the ion energy or the magnetic field strength is varied, and where the mass filter allows only one mass/charge ratio to be transmitted and a spectrum can be recorded by scanning through the mass range.
One type of mass spectrometer is a mass spectrograph. In a mass spectrograph the ions are spatially separated in a magnetic field and detected with a position sensitive detector. The concept of a double focusing mass spectrograph was first introduced by Maftauch and Herzog (MH) in 1940 (J. Mattauch, Ergebnisse der exakten Naturwissenschaften, vol. 19, pp. 170-236, 1940).
Double focusing refers to an instrument's ability to refocus both the energy spread as well as the spatial beam spread. Modern developments in magnet and micro machining technologies allow dramatic reductions in the size of these instruments. The length of the focal plane in a mass spectrometer capable of VOC and semi-VOC analysis is reduced to a few centimeters.
The typical specifications of a small confocal plane layout Maftauch-Herzog instrument are summarized below:
Electron impact ionization, Rhenium filament
DC-voltages and permanent magnet
Ion Energy: 0.5-2.5 kV DC
Mass Range: 2-200 D
Faraday cup detector array or strip charge detector
Integrating operational amplifier with up to 1011 gain
Duty Cycle: >99%
Read-Out time: 0.03 sec to 10 sec
Sensitivity: approximately 10 ppm with strip charge detector
In addition, the ion optic elements are mounted in the vacuum chamber floor or on chamber walls. The optics can also be an integral part of the vacuum housing. In small instruments, however, the ion optic elements can be built on a base plate that acts as an “optical bench.” This bench supports the ion optic elements. The base plate is mounted against a vacuum or master flange to provide a vacuum seal needed to operate the mass spectrometer under vacuum. The base plate can also function as the vacuum or master flange itself.
A Mattauch-Herzog ion detector is a position sensitive detector. Numerous concepts have been developed over the last decades. Recent developments focus on solid state based direct ion detection as an alternative to previously used electro optical ion detection (EOID).
The electro optical ion detector (EOID) converts the ions in a multi-channel-plate (MCP) into electrons, amplifies the electrons (in the same MCP), and illuminates a phosphorus film bombarded with the electrons emitted from the MCP. The image formed on phosphorus film is recorded with a photo diode array via a fiber optic coupler. This type of EOID is described in detail in U.S. Pat. No. 5,801,380. The EOID is intended for the simultaneous measurement of ions spatially separated along the focal plane of the mass spectrometer. The EOID operates by converting ions to electrons and then to photons. The photons form images of the ion-induced signals. The ions generate electrons by impinging on a microchannel electron multiplier array. The electrons are accelerated to a phosphor-coated fiber-optic plate that generates photon images. These images are detected using a photodetector array.
According to a different configuration, a direct charge measurement can be based on a micro-machined Faraday cup detector array. Here, an array of individually addressable Faraday cups monitors the ion beam. The charge collected in individual elements of the array is handed over to an amplifier via a multiplexer unit. This layout reduces the number of amplifiers and feedthroughs needed. This concept is described in detail in recent publications, such as “A. A. Scheidemann, R. B. Darling, F. J. Schumacher, and A. Isakarov, Tech. Digest of the 14th Int Forum on Process Analytical Chem. (IFPAC-2000), Lake Las Vegas, Nev., Jan. 23-26, 2000, abstract 1-067”; “R. B. Darling, A. A. Scheidemann, K. N. Bhat, and T.-C. Chen., Proc. of the 14th IEEE Int. Conf on Micro Electro Mechanical Systems (MEMS-2001), Interlaken, Switzerland, Jan. 21-25, 2001, pp. 90-93”; and Non-Provisional patent application Ser. No. 09/744,360 titled “Charged Particle Beam Detection System.”
Other important references regarding spectrometers are Nier, D. J. Schlutter, Rev. Sci. Instrum. 56(2), pp. 214-219, 1985; “Fundamentals of Focal Plane Detector cs” K. Birkinshaw Jrnl. of Mass Spectrometry, Vol. 32,795-806 (1997); and T. W. Burgoyne et. al., J. Am. Soc. Mass Spectrum 8, pp. 307-318, 1997.
Alternatively, especially for low energy ions, a flat metallic strip (referred to as a strip charge detector (SCD)) on a grounded and insulated background can be used with an MCP. As described above, an MCP converts ions into electrons and amplifies the electrons. The SCD detects the electrons and generates a charge. Again the charge is handed over to an amplifier via a multiplexer.
Another embodiment of an ion detector array is disclosed in U.S. Pat. No. 6,576,899 and is referred to as a shift register based direct ion detector.
The shift register based direct ion detector defines a charge sensing system that can be used in a GC/MS system, with a modification to allow direct measurement of ions in the mass spectrometer device without conversion to electrons and photons (e.g., EOID prior to measurement). The detector may use charge coupled device (CCD) technology with metal oxide semiconductors. The GC/MS system may use direct detection and collection of the charged particles using the detector. The detected charged particles form the equivalent of an image charge that directly accumulates in a shift register associated with a part of the CCD. This signal charge can be clocked through the CCD in a conventional way, to a single output amplifier. Since the CCD uses only one charge-to-voltage conversion amplifier for the entire detector, signal gains and offset variations of individual elements in the detector array are minimized.
A Mattauch-Herzog detector array, which can be composed of a Faraday cup detector array, a strip charge detector, or another type of the aforementioned detectors, is placed at the exit end of the magnet, which is commonly designed to be coplanar with the focal plane of the device.
The resolution of the Mattauch-Herzog instrument is governed, among other quantities, by a width of an object slit and the spatial resolution of the detector. Thus one desires to make the object slit as narrow as possible. However, the total ion current leaving the ionizer corresponds to an area (i.e., size) of the object slit. Thus too small of an object slit may lead to ion currents which are too small for practical applications. Typically, object slit sizes from 0.1 to 0.01 mm are desirable. A detailed discussion of the Mattauch-Herzog equation is given in “Nier, D. J. Schlutter, Rev. Sci. Instrum. 56(2), pp. 214-219, 1985; and T. W. Burgoyne et. al., J. Am. Soc. Mass Spectrum 8, pp. 307-318, 1997.
FIG. 1 shows a GC/MS instrument 100. The instrument 100 includes a Mattauch-Herzog double focusing MS 110 assembled with a GC 150. The MS 110 includes an ionizer 114, a shunt and aperture 116, an electrostatic energy analyzer 118, a magnetic section 120, and a focal plane section 122 (also referred to as a detector).
In operation of the MS 110, a gaseous material or a vapor is introduced into the ionizer 114, either directly or through the GC 150 (for complex mixtures or compounds). The material is bombarded by electrons to produce ions. The ions are focused in the shunt and aperture section 116 to form an ion beam 124. The ions are separated according to their charge/mass ratio as they move through the electrostatic energy analyzer 118 and the magnetic section 120. The ions are then detected in the focal plane section 122, as described in U.S. Pat. No. 5,801,380. The ion separation process takes place under a vacuum pressure on the order of about 10−5 Torr, which can be achieved with a vacuum pump (not shown).
The GC 150 includes a sample injector valve V, which has an entry port S for introduction of the sample, and an exit port W for the waste after the sample has been vaporized and/or decomposed, typically by heat. The sample injector valve V may be a liquid injector. The part to be analyzed, referred to as analyte is carried by a carrier gas, such as dry air, hydrogen, or helium, for example, to a capillary microbore column M (wall coated open tubular, or porous layer open tubular, or packed, etc.), where its constituents are separated by different absorption rates on the wall of the microbore column M. The microbore column M has a rather small inside diameter, of the order of about 50-500 μm in the illustrated embodiment. The carrier gas flow rate is about 0.2 to 5 atm. cm3/sec, although higher flow rates, for example 20 atm. cm3/sec, are possible.
A larger microbore column M bore requires a larger vacuum pump, whereas a smaller bore produces narrower peaks of the effluent, which may result in a loss of signal. In general, the gas flow rate is a function of the inner diameter, the length of the column M, the pressure of the carrier gas, and the temperature of the carrier gas. The width of the peak again is a function of the injection time, the stationary phase of the column (e.g., polarity, film thickness, distribution in the column), the width and length of the column, the temperature and the gas velocity. One method of determining a size of the microbore column M bore is addressed in U.S. Pat. No. 6,046,451.
Patents representing major advances in the art of mass spectrometers and gas chromatographs/mass spectrometers are U.S. Pat. Nos. 5,317,151; 5,801,380; 6,046,451; 6,182,831; 6,191,419; 6,403,956; 6,576,899; and 6,847,036. Also U.S. patent application Ser. Nos. 10/811,576 and 10/860,776.